Chapter 2. API Reference
xTimerReset(xTimer, xTicksToWait)
BaseType_t xTimerReset( TimerHandle_t xTimer, TickType_t xTicksToWait );
Timer functionality is provided by a timer service/daemon task. Many of the public FreeRTOS timer API
functions send commands to the timer service task through a queue called the timer command queue. The
timer command queue is private to the kernel itself and is not directly accessible to application code. The
length of the timer command queue is set by the configTIMER_QUEUE_LENGTH configuration constant.
xTimerReset() re-starts a timer that was previously created using the xTimerCreate() API function. If the
timer had already been started and was already in the active state, then xTimerReset() will cause the timer
to re-evaluate its expiry time so that it is relative to when xTimerReset() was called. If the timer was in the
dormant state then xTimerReset() has equivalent functionality to the xTimerStart() API function.
Resetting a timer ensures the timer is in the active state. If the timer is not stopped, deleted, or reset in the
mean time, the callback function associated with the timer will get called ‘n’ticks after xTimerReset() was
called, where ‘n’is the timers defined period.
It is valid to call xTimerReset() before the scheduler has been started, but when this is done the timer will not
actually start until the scheduler is started, and the timers expiry time will be relative to when the scheduler is
started, not relative to when xTimerReset() was called.
The configUSE_TIMERS configuration constant must be set to 1 for xTimerReset() to be available.
Example usage:
* // When a key is pressed, an LCD back-light is switched on. If 5 seconds␣
,→pass
* // without a key being pressed, then the LCD back-light is switched off. In
* // this case, the timer is a one-shot timer.
*
* TimerHandle_t xBacklightTimer = NULL;
*
* // The callback function assigned to the one-shot timer. In this case the
* // parameter is not used.
* void vBacklightTimerCallback( TimerHandle_t pxTimer )
* {
* // The timer expired, therefore 5 seconds must have passed since a key
* // was pressed. Switch off the LCD back-light.
* vSetBacklightState( BACKLIGHT_OFF );
* }
*
* // The key press event handler.
* void vKeyPressEventHandler( char cKey )
* {
* // Ensure the LCD back-light is on, then reset the timer that is
* // responsible for turning the back-light off after 5 seconds of
* // key inactivity. Wait 10 ticks for the command to be successfully sent
* // if it cannot be sent immediately.
* vSetBacklightState( BACKLIGHT_ON );
* if( xTimerReset( xBacklightTimer, 100 ) != pdPASS )
* {
* // The reset command was not executed successfully. Take appropriate
* // action here.
* }
*
* // Perform the rest of the key processing here.
* }
*
* void main( void )
* {
* int32_t x;
*
* // Create then start the one-shot timer that is responsible for turning
* // the back-light off if no keys are pressed within a 5 second period.
(continues on next page)
Espressif Systems 925
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